A shock absorber was discussed in JP 2008-215459 A. This shock absorber includes a cylinder, a piston slidably inserted into the cylinder to partition the cylinder into an expanding-side chamber and a contracting-side chamber, a damping passage that connects the expanding-side chamber with the contracting-side chamber provided in the piston, a housing provided in a leading end of the piston rod to form a pressurizing chamber, a free piston slidably inserted into the pressurizing chamber to partition the pressurizing chamber into an expanding-side pressurizing chamber and a contracting-side pressurizing chamber, a coil spring that biases the free piston, an expanding-side passage that connects the expanding-side chamber with the expanding-side pressurizing chamber, and a contracting-side passage that connects the contracting-side chamber with the contracting-side pressurizing chamber.
Since the pressurizing chamber is partitioned by the free piston into the expanding-side pressurizing chamber and the contracting-side pressurizing chamber, the expanding-side chamber and the contracting-side chamber do not directly communicate with each other through the expanding-side passage and the contracting-side passage. However, as the free piston moves, a volume ratio between the expanding-side chamber and the contracting-side chamber changes, so that a liquid inside the pressurizing chamber accesses the expanding-side chamber and the contracting-side chamber depending on the movement amount of the free piston. Therefore, the pressurizing chamber appears such that the expanding-side chamber and the contracting-side chamber communicate with each other through the expanding-side passage and the contracting-side passage.
Such a shock absorber generates a strong damping force for a low frequency vibration input, and generates a weak damping force for a high frequency vibration input. As a result, the shock absorber may generate a strong damping force when the input vibration frequency is low, for example, when a vehicle turns, and may generate a weak damping force when the input vibration frequency is high, for example, when a vehicle travels on an uneven road surface. Therefore, the shock absorber can improve ride quality of a vehicle.